1. Field of the Invention
The present invention relates in general to the field of electronics, and more specifically to a method and system for providing thermal management in a lighting system using multiple, controlled power dissipation circuits.
2. Description of the Related Art
The development and use of energy efficient technologies continues to be a high priority for many entities including many companies and countries. One area of interest is the replacement of incandescent lamps with more energy efficient lamps such as lamps based on electronic light sources. For this description, electronic light sources are light emitting diodes (LEDs) or compact fluorescent lamps (CFLs). As subsequently discussed with reference to FIG. 4, electronic light sources use significantly less energy per lumen of light output than incandescent lamps. In other words, for a given amount of energy, an LED and a CFL are much brighter than an incandescent lamp.
The development of electronic light source based lamps and are not without many challenges. One of the challenges is developing electronic light source based lamps that are compatible with existing infrastructure. Another challenge is dissipating heat in electronic light source based lamps. The following discussion focuses on LED-based lighting systems but is also applicable to CFL-based lighting systems and combination LED and CFL based lighting systems.
FIG. 1 depicts an exemplary LED-based lamp 100 that at least attempts compatibility with the existing infrastructure. The lamp 100 includes a threaded body 102 designed for compatibility with existing incandescent lamp sockets. In other embodiments, the lamp 100 simply has a 2-wire or 3-wire connector (not shown). The lamp 100 receives power from a voltage source 104. Voltage source 104 provides an input supply voltage VIN, which is, for example, a nominally 60 Hz/110 V line voltage in the United States of America or a nominally 50 Hz/220 V line voltage in Europe. Dimmer 102 phase cuts the input supply voltage VIN to generate the phase cut input voltage VφR_IN. Incandescent lamps operate directly from the phase cut input voltage VφR_IN. However, the LEDs 108 operate from an approximately constant output voltage VLD and draw an approximately constant current iLED for a given brightness.
The lamp 100 includes power conversion circuitry 110 to convert the phase cut input voltage Vφ_IN and the dimmer input current iDIM into the output voltage VLD and output current iLED utilized by LEDs 108. The power conversion circuitry 110 includes an interface 112 to rectify the phase cut input voltage Vφ_IN and provide electromagnetic interference (EMI) protection. A boost-type switching power converter 114 converts the phase cut input voltage Vφ_IN into a regulated, approximately constant link voltage VLINK. A transformer circuit 116 converts the link voltage VLINK into the output voltage VLD. The power conversion circuitry 110 also includes a controller 118 to control the conversion of power by the switching power converter 114 and transformer circuit 116. A lens 120 encloses the LEDs 108 for protection and light diffusion.
The LEDs 108 generate heat, and heat can degrade and shorten the life span of the LEDs 108. To help manage the heat generated by the LEDs 108, the lamp 100 includes a heat sink 122 that surrounds the LEDs 108 and provides conductive cooling.
The power conversion circuitry 110 also generates heat during operation. Many of the components of lamp 100 become static heat sources. Additionally in some instances, as subsequently discussed in more detail, the Power In to the lamp 100 exceeds the Power Out demands of the LEDs 108 plus any inherent power losses. Switching power converters convert power received from a power source, such as a voltage supply, into power suitable for a load. The power received from the voltage supply 104 is referred to as “POWER IN”, and the power provided to the LEDs 108 is referred to as “POWER OUT”. All switching power converters have some inherent power losses due to, for example, non-ideal component characteristics. Such inherent power losses tend to be minimized so as to increase the efficiency of the switching power converters. Inherent power losses are represented herein by “PINH”. In some contexts, the amount of power supplied to the switching power converter can exceed the amount of power provided by the switching power converter to a load, i.e. POWER IN>POWER OUT+PINH. When the POWER IN is greater than the POWER OUT+PINH, the boost switching power converter 114 passively dissipates the excess energy using the passive, power dissipation resistor 126.
A dimmable lighting system that includes a low power lamp, such as one or more light emitting diodes (LEDs), represents one context when the POWER IN to the switching power converter can be greater than the POWER OUT PINH of the switching power converter. In this exemplary context, the switching power converter receives current through a triode for alternating current (“triac”) based dimmer. Once a triac-based dimmer begins conducting during a cycle of an alternating current (“AC”) supply voltage to prevent the triac from disadvantageously, prematurely disconnecting during mid-cycle of the supply voltage, the switching power converter draws a minimum current referred to as a “hold current”. As long as an input current to the switching power converter is greater than or equal to the hold current, the triac-based dimmer should not prematurely disconnect. For a leading edge dimmer, a premature disconnect occurs when the dimmer begins conducting and stops conducting prior to reaching a zero crossing of the supply voltage. Premature disconnects can cause problems with the lighting system, such as flicker and instability.
Thus, to prevent premature disconnection of the triac-based dimmer, the minimum POWER IN to the switching power converter equals the hold current (“iHOLD”) times an input voltage “VIN” to the switching power converter. Conventional triac-based dimmers were designed to provide power to incandescent light bulbs. For desired dimming levels, an incandescent light bulb generally draws a current at least equal to the hold current for all usable dimming levels. However, other lamps, such as LEDs are more efficient than incandescent light bulbs in terms of power versus light output and, thus, provide equivalent light output while using less power than an incandescent light bulb. Thus, lighting systems with LEDs typically utilize less power and less current than incandescent bulbs. To balance the power when the lighting system draws more POWER IN than the lighting system inherently dissipates and provides as POWER OUT to the lamp, the lighting system utilizes one or more passive resistors to internally dissipate excess power.
Thus, in addition to heat generated by lamp 100 during normal operation, the lamp 100 dissipates excess energy in the form of heat represented by the difference between the POWER IN and POWER OUT+PINH. The power conversion circuitry 110 includes a power dissipation resistor 126 to dissipate the excess energy in the form of heat. The power conversion circuitry 110 is generally surrounded by stabilizing material 124, such as potting compound, 124 to provide structural support. However, the potting compound tends to thermally insulate the power conversion circuitry 110. Thus, the energy dissipated by power conversion circuitry 110 including the power dissipation resistor 126 tends to remain statically concentrated.
FIGS. 2, 3, and 4 describe power conversion circuitry in a lighting system in more detail. FIG. 2 depicts a lighting system 200 that includes a leading edge, phase-cut dimmer 202. FIG. 3 depicts ideal, exemplary voltage graphs 300 associated with the lighting system 200. Referring to FIGS. 2 and 3, the lighting system 200 receives an AC supply voltage VIN from voltage supply 104. The leading edge dimmer 102 phase cuts leading edges, such as leading edges 304 and 306, of each half cycle of supply voltage VIN. Since each half cycle of supply voltage VIN is 180 degrees of the input supply voltage VIN, the leading edge dimmer 202 phase cuts the supply voltage VIN at an angle greater than 0 degrees and less than 180 degrees. Generally, the voltage phase cutting range of a leading edge dimmer 202 is 20 degrees to 170 degrees. “Phase cutting” the supply voltage refers to modulating a leading edge phase angle of each cycle of an alternating current (“AC”) supply voltage. “Phase cutting” of the supply voltage is also commonly referred to as “chopping”. Phase cutting the supply voltage reduces the average power supplied to a load, such as a lighting system, and thereby controls the energy provided to the load.
The input signal voltage Vφ_IN to the lighting system 200 represents a dimming level that causes the lighting system 200 to adjust power delivered to a lamp 222, and, thus, depending on the dimming level, increase or decrease the brightness of the lamp 222. Many different types of dimmers exist. In general, dimmers use a digital or analog coded dimming signal that indicates a desired dimming level. For example, the triac-based dimmer 202 phase cuts a leading edge of the AC input supply voltage VIN. The leading edge dimmer 202 can be any type of leading edge dimmer such as a triac-based leading edge dimmer available from Lutron Electronics, Inc. of Coopersberg, Pa. (“Lutron”). A triac-based leading edge dimmer is described in the Background section of U.S. patent application Ser. No. 12/858,164, entitled Dimmer Output Emulation, filed on Aug. 17, 2010, and inventor John L. Melanson.
The phase cut dimmer 202 supplies the input voltage Vφ_IN as modified by the phase cut dimmer 202 to a full bridge diode rectifier 206. The full bridge rectifier 206 supplies an AC rectified voltage VφR_IN to the switching power converter 208. Capacitor 220 filters high frequency components from rectified voltage VφR_IN. To control the operation of switching power converter 208, controller 220 generates a control signal CS0 to control conductivity of field effect transistor (FET) switch 212. The control signal CS0 is a pulse width modulated signal. Control signal CS0 waveform 214 represents an exemplary control signal CS0. The controller 220 generates the control signal CS0 with two states as shown in the waveform 214. Each pulse of control signal CS0 turns switch 212 ON (i.e. conducts) represents a first state that causes the switch 212 to operate efficiently and minimize power dissipation by the switch 212. During each pulse of control signal CS0, the inductor current iL increases, as shown in the exemplary inductor current waveform 215, to charge inductor 216 during a charging phase TC. Diode 218 prevents current flow from link capacitor 220 into switch 212. When the pulse of control signals CS0 ends, the control signal CS0 is in a second state, and the inductor 216 reverses voltage polarity (commonly referred to as “flyback”). The inductor current iL decreases during the flyback phase TFB, as shown in inductor current waveform 215. The inductor current iL boosts the link voltage across the link capacitor 220 through diode 218. When the flyback phase TFB ends and when the next charging phase TC begins depends on the operating mode of the switching power converter. In discontinuous conduction mode (DCM), the flyback phase TFB ends before the next charging phase TC begins. However, regardless of whether the switching power converter 208 operates in discontinuous conduction mode, continuous conduction mode, or critical conduction mode, the flyback phase TFB begins as soon as the charging phase TC ends.
The switching power converter 208 is a boost-type converter, and, thus, the link voltage VLINK is greater than the rectified input voltage VφR_IN. Controller 220 senses the rectified input voltage VφR_IN at node 224 and senses the link voltage VLINK at node 226. Controller 220 operates the switching power converter 208 to maintain an approximately constant link voltage VLINK for lamp 222, provide power factor correction, and correlate the link current iLINK with the phase cut angle of the rectified input voltage VφR_IN. Lamp 222 includes one or more LEDs or CFLs.
FIG. 3 depicts an exemplary light output/power graph 800 that compares light output per watt of power for an exemplary incandescent bulb and an exemplary light emitting diode (LED). Per watt of power, LEDs provide more light output than incandescent light bulbs. The low power usage by LEDs correlates to a relatively low operating current compared to the operating current for an incandescent light bulb. Since the light output of LEDs is approximately linear with power and LEDs operate at an approximately constant voltage, operating current for an LED decreases approximately linearly with decreasing light output and power.
Referring to FIGS. 2, 3, and 4, to decrease the light output of the lamp 222, the phase cut dimmer 202 increases the phase cut angle of the rectified input voltage VφR_IN, i.e. time TOFF increases and time TON decreases. The controller 220 responds to the increased phase cut angle by decreasing the current iLINK provided to the lamp 222, which decreases the light output of the lamp 222.
The switching power converter 208 includes a power dissipation resistor 228 so that the dimmer current iDIM does not fall below the hold current value and prematurely disconnect during a cycle of the rectified input voltage VφR_IN. The “POWER IN” supplied to the switching power converter 208 equals Vφ_IN·iDIM. The “POWER OUT” supplied by switching power converter 208 equals VLINK·iLINK. Because of the relatively low power requirements of an LED based lamp 222, particularly at low light output levels, if the POWER IN equals the POWER OUT+PINH, the dimmer current iDIM may fall below the hold current value and cause the phase-cut dimmer 202 to prematurely disconnect. In this situation, to prevent the dimmer current iDIM from falling below the hold current value, the controller 220 causes the switching power converter 208 to maintain the dimmer current iDIM above the hold current value, which causes the POWER IN to be greater than the POWER OUT+PINH. Since the POWER IN is greater than the POWER OUT+PINH, the switching power converter 208 dissipates the excess power through power dissipation resistor 228.
Because of component non-idealities, the switching power converter 208 includes inherent power losses. Inherent power losses include conductor resistances and switching losses in switch 212. However, circuits are generally designed to minimize inherent power losses, and these inherent power losses are often negligible and, thus, insufficient to dissipate enough power to compensate for the difference between the POWER IN and the POWER OUT+PINH at some POWER OUT levels. To increase the power loss of switching power converter so that the dimmer current iDIM remains above a hold current value even at lower power demand by the lamp 222, switching power converter 208 includes the resistor 228 to create a passive power loss when switch 212 conducts the inductor current iL. For negligible inherent power losses, the resistance value of the resistor 228 is selected so that when the switching power converter is providing a minimum link current iLINK, the POWER IN=POWER OUT+PINH+PASSIVE POWER DISSIPATE.
Resistor 228 is relatively cheap to implement as part of switching power converter 208. However, when the link current iLINK is sufficiently high such that POWER IN equals POWER OUT+PINH, the dimmer input current iDIM could be maintained above the hold current value without dissipating power through resistor 228. However, since the dimmer input current iDIM always flows through the resistor 228 when the switch 208 is conducts, the resistor 228 still passively dissipates power regardless of whether the POWER IN is equal to the POWER OUT+PINH, which decreases the efficiency of lighting system 200.